Originally, "mace" referred to a club with a weight on the end. What you're referring to was known as a flail. Although I think video games have made this misnomer more widely known than the correct terminology.

I think it depends on why they broke up. If the ex has reason to think he's dangerous or something, then maybe she made the right decision. One of my exes was very abusive, if he tried to talk to me normally I would punch him and leave.

I wonder if perhaps OP was abusive and so now she finds his mere presence threatening. Not likely, since she "kept walking" rather than ran away, but it's a possibility if she's overconfident in the effects of mace.